1. Field of the Invention
The present invention relates to a serial optical signal distribution system and method for superfast architectures (VLSI), protected against timing problems related to waveform distortions.
The present invention also relates to an optical/electrical converter for implementing the serial optical signal distribution system, and capable of generating an electric signal, in particular but not exclusively an electric clock signal, in response to two trains of light pulses. The leading edges of the light pulses of the first train correspond to the leading edges of the pulses of the electrical signal. The leading edges of the light pulses of the second train correspond to the trailing edges of the pulses of the electrical clock pulses.
2. Brief Description of the Prior Art
The speed of state-of-art VLSI circuits and systems is limited primarily by their interconnects. In particular, clock timing constraints introduce the most critical limitations for the performance of VLSI circuits and systems. Usually, several clock signals, derived from a single master clock and often characterized by a high fan-out, arrive at different, sometimes distant locations. Obviously, the VLSI circuits or systems will not operate satisfactorily if the phase between the same clock signals arriving at different locations varies, and if phase differences between different clock signals are unstable.
With increasing clock frequency, clock skew caused by variation in propagation time through passive and active elements of the VLSI circuits or systems, increases. The contribution of the active elements to clock skew results from the variations of many technological parameters as well as from the variations of thresholds, power supply voltage, temperature, etc. The contribution of the passive elements to clock skew includes signal distortions in electrical transmission lines due to reflections, crosstalk, phase dispersion, and ground loop impedance.
Clock skew, from both origins, can be if not completely eliminated, greatly reduced if the clock signals are distributed through optical links, while keeping electrical interconnection lines as short as possible, and by eliminating reshaping of the electric clock signal.
The non-conductive nature of optical waveguides eliminates many of the above mentioned problems associated with conductive transmission lines. Introduction of optical waveguides therefore improve immunity of the circuits or systems to the above mentioned effects. For that reason, several optical clock distribution architectures have been proposed in the past.
An excellent optical clock distribution architecture for high-performance computer system is presented in U.S. Pat. No. 4,959,540 (Fan et al.) issued on Sep. 25, 1990. To produce clock signals having a predetermined frequency, pulse width and relative phase, Fan et al. use a remote laser clock generator to produce a train of ultrashort pulses. The laser pulses are propagated through an optical fiber, and then equally divided into 2N optical fibers by means of a beam splitter. The optical fibers have different lengths, resulting in different propagation times of the optical pulses. The delayed optical trains of laser pulses are grouped in pairs, and each pair is converted to a corresponding train of short electrical pulses by means of a bistable (toggle) element.
The major disadvantage of the architecture of Fan et al. is that the optical signals are transmitted in parallel, which results in a huge number of optical waveguides which have to be coupled for example to an integrated circuit (IC) or a multichip module (MCM). Another disadvantage is the number of connections necessary between the laser clock generator and the IC or MCM.